This invention relates to clays and is particularly concerned with delaminated clays which contain both micropores and macropores and can be used as a constituent of various types of hydrocarbon conversion catalysts.
Smectite clays are a well known class of naturally occurring and synthetic layered clays which swell or expand when exposed to moisture. The particles of the swelling clays are composed of platelets stacked one on top another to form aggregates in which alkali metal and alkaline earth metal cations are interposed between the platelets. Each platelet can be visualized as a sandwich with the two outer layers composed primarily of silicon in tetrahedral coordination and the inner layer of a different element depending on the actual species of smectite clay. For example, the inner layer of a platelet in a montmorillonite clay is composed of aluminum in octahedral coordination, whereas in a hectorite clay the octahedral sites in the inner layer are occupied by magnesium and lithium. The stacked arrangement of platelets normally has a repeating structure about every 9 Angstroms. Besides montmorillonite and hectorite mentioned above, other types of smectite clays include beidellite, nontronite, and saponite. Smectite clays are discussed in detail in the book entitled Clay Mineralogy, second edition, authored by Ralph E. Grim and published by the McGraw-Hill Book Company in 1968, the disclosure of which book is hereby incorporated by reference in its entirety.
It is known that the platelets which comprise a swelling smectite clay can be separated by as much as 6 to 10 Angstroms by intercalating thermally stable, robust, three dimensional cations between the platelets of the clay. Clays modified in this manner are referred to as pillared clays or cross-linked smectites. The size, shape and nature of the intercalated cations allow them to impart acidity to the clay while serving as pillars to prop apart the layers of the clay, thereby exposing the surface of the layers for catalytic reactions. The fairly homogeneous distribution of pillars in the interlayered spaces of the clay form an array of rectangular openings, typically about 8 by 15 Angstroms in size, which enable the pillared clay to behave like a two dimensional sieve. By adjusting the size of the intercalated cations or the spacing between such cations or both, the pore size of the pillared clay may be adjusted to suit a particular application.
Pillared clays are typically prepared by reacting a smectite clay, such as montmorillonite, with polyoxymetal cations such as polyoxycations of aluminum and zirconium. The reaction product is normally dried in air and calcined to convert the intercalated cations into metal oxide clusters interposed between the platelets of the clay such that the spacing between the platelets ranges from about 6 to about 10 Angstroms and is maintained at such values when the clay is heated to a temperature between about 500.degree. C. and 700.degree. C. When the reaction product is dried, the clay platelets, which are propped apart by the metal oxide clusters, orient themselves face-to-face, thereby forming a lamellar structure which yields an X-ray diffraction pattern containing a distinct first order or (001) reflection. The extent of lamellar ordering is indicated by the X-ray powder diffraction pattern of the pillared clay. A well-ordered, air-dried, pillared montmorillonite may exhibit six or more orders of reflections whereas a freeze dried pillared montmorillonite may show only one or two orders of reflection. Pillared clays and their preparation are described more fully in the article entitled "Intercalated Clay Catalysts," Science, Vol. 220, No. 4595 pp. 365-371 (April 22, 1983) and in U.S. Pat. Nos. 4,367,163, 4,271,043, 4,248,739, 4,238,364, 4,216,188 and 4,176,090. The disclosures of the aforementioned article and patents are hereby incorporated by reference in their entireties.
The well-ordered stacking of platelets in natural and pillared montmorillonites can be directly observed with a transmission electron microscope (TEM). The resultant TEM micrographs show an extensive, long range, face-to-face platelet stacking in natural montmorillonite and indicate that this well-ordered orientation of platelets is retained after the clay has been reacted with polyoxyaluminum cations to form a pillared clay whose X-ray diffraction pattern contains a distinct first order reflection.
It has been reported in the literature that Laponite clay, a synthetic hectorite, can be reacted with polyoxyaluminum cations to form a flocculated reaction product which, when freeze dried, forms a clay whose platelets are unordered as compared to those of a pillared clay. In other words, in addition to containing platelets oriented face-to-face in an ordered, lamellar structure, the freeze dried clay product, which is known as a delaminated clay, also contains platelets which are oriented edge-to-edge and edge-to-face, thereby forming macropores of the type found in amorphous aluminosilicate supports. FIG. 1 contains schematic representations of a pillared clay and a delaminated clay which illustrate the different orientation of the platelets in these two types of clays. As can be seen, the pillared clay is a well ordered structure in which the clay platelets are propped apart by pillars or metal oxide clusters, represented by dark dots in the figure, and are substantially all oriented face-to-face in a stacked fashion. On the other hand, the schematic representation of the delaminated clay shows many platelets oriented edge-to-edge and edge-to-face, thereby generating a "house-of-cards" structure containing macropores of a size typically found in amorphous aluminosilicates in addition to the micropores found in pillared clays. Platelet aggregates having a "house-of-cards" structure are described by van Olphen in the book entitled Clay Colloid Chemistry published by Interscience Publishers in 1963, the disclosure of which book is hereby incorporated by reference in its entirety. The combination of macroporosity and microporosity found in delaminated clays leads to desirable catalytic properties that cannot be obtained with pillared clays alone.
Because of the rather random orientation of platelets in a delaminated clay, the X-ray diffraction pattern of such clays, unlike that of a pillared clay, will not contain a distinct first order or (001) reflection. An example of this can be seen in FIG. 2 which shows the X-ray diffraction pattern of a pillared montmorillonite prepared by air drying the flocculated product obtained by reacting montmorillonite with polyoxyaluminum cations. As can be seen, its X-ray diffraction pattern contains a first order reflection at a two-theta value of about 4.5 degrees. In contrast, the X-ray diffraction pattern of a delaminated Laponite clay prepared by freeze drying the flocculated reaction product obtained when Laponite B clay is reacted with polyoxyaluminum cations does not exhibit a first order reflection.
The preparation of delaminated clays by reacting Laponite clay with polyoxycations of aluminum and subsequently freeze drying the flocculated reaction product is discussed in the chapter entitled "Preparation and Properties of Pillared and Delaminated Clay Catalysts," authored by T. J. Pinnavaia and appearing in the book entitled Heterogeneous Catalysis, edited by B. L. Shapiro and published by the Texas A&M University Press, College Station, Tex., page 142 (1984) and in the article entitled "On the Pillaring and Delamination of Smectite Clay Catalysts by Polyoxo Cations of Aluminum," authored by T. J. Pinnavaia, M. S. Tzou, S. D. Landau, and R. H. Raythatha, and appearing at page 195 in the Journal of Molecular Catalysis, Vol. 27, (1984). The disclosures of these two publications are hereby incorporated by reference in their entireties. As pointed out in the latter mentioned article appearing in the Journal of Molecular Catalysis, delaminated clays having X-ray diffraction patterns with no distinct first order reflection have been prepared only by freeze drying the flocculated reaction product. Unfortunately, the use of freeze drying to prepare delaminated clays is very expensive in that it requires special equipment and long periods of time, typically as long as 15 days. Air drying of the flocculated reaction product, on the other hand, can be implemented in many existing catalyst plants in less than 24 hours. Obviously, it would be highly desirable to produce delaminated clays whose X-ray diffraction patterns contain no distinct first order reflection using an air drying step instead of freeze drying.
Accordingly, it is one of the objects of the present invention to provide a method for producing delaminated clays without the necessity of using a freeze drying step. It is another object of the invention to produce delaminated clays using polyoxymetal cations other than polyoxyaluminum cations. It is yet a further object of the invention to provide catalyst compositions containing such delaminated clays and hydrocarbon conversion processes utilizing such catalyst compositions. These and other objects of the invention will become more apparent in light of the following description of the invention.